This invention relates in general to television receiver voltage regulators, and in particular to a voltage regulator for permitting a television receiver to operate free from video display distortion caused by input voltage excursions.
Effective voltage regulation in a television receiver is essential for optimum receiver operation and display of video information. An excessively large increase in input voltage could result in component damage and display degradation due to arcing in such components as the deflection yoke or picture tube. High voltage conditions may also cause excessive radiation from the cathode ray tube. Large input voltage excursions on the low end cause video display degradation in the form of objectionable diminishing of the video image size. In addition, low input line voltages cause excessive distortion and break-up of the video display due to excessive ripple of the input signal.
The prior art discloses various attempts to precisely regulate the input voltage to a television receiver in order to avoid these problems. One approach is described in U.S. Pat. No. 3,641,267 to Cavallari directed to a voltage-step-down circuit for a television receiver for minimizing the effect of wide variations in input voltage on the output signal. The circuit includes a Zener diode in the input circuit of either the main chopping transistor, which is normally blocked and is used in driving the sweep circuits of the television receiver, or an ancillary transistor connected ahead of the main transistor in cascade therewith. This arrangement limits the base current and therefore the collector current of the associated transistor thereby limiting the amplitude of the pulse to be integrated. Where the integrating network includes the primary winding of a transformer with a secondary winding driving the chopping transistor, the current through the primary winding may vary inasmuch as any excess secondary current as dissipated through the Zener diode connected between the base and emitter of the associated transistor. As a result, any change in load current imparts only a negligible effect upon the integrated output voltage. Even when thus stabilized, however, this system is capable of developing transient voltages producing ripple distortion in the video presentation and possibly even leading to breakdown of the chopping transistor.
Another related system for stabilizing the DC input provided to a television receiver is disclosed in U.S. Pat. No. 3,629,497 to Soardi et al. This invention is intended to improve the previously described patent in that the collector current of the first transistor must necessarily pass through the emitter of the second transistor, in contrast to the operation of the previously discussed patent in which the second transistor is provided with its own independent input circuit from another transformer secondary. In Soardi the first transistor in its blocked condition effectively cuts off the second transistor and insulates it from any transient voltage appearing on the input to the chopper network. The insertion of a high value resistor in the conductive connection between the bases of these two transistors allegedly prevents any discharge of the capacitive branch of the interstage coupling network through the emitter and base of the second transistor. By thus coupling both chopping transistors to a single transformer winding the power supply output is apparently stabilized against fluctuations in input drive voltage. Thus, this invention is concerned with the stability of the DC output level and fails to address video display distortion caused by AC components of a fluctuating, low direct voltage input signal.
Still another approach to DC input level control in a television receiver is described in U.S. Pat. No. 4,028,726 to Argy wherein is described an improved image raster expanding system for a television receiver. This system combines horizontal and vertical overscan blanking with enhanced contrast and color level control in compensating for deterioration in contrast in brightness levels of the image commonly experienced in raster expansion. By adjusting the horizontal and vertical overscan blanking pulses to optimize picture tube brightness and contrast in blanking those portions of the image which are beyond the image screen, this system is capable of providing a non-degraded video display in the raster expended, or "zoom", mode of operation. This reference is cited not because it attempts to stabilize the signal provided to television circuitry in the presence of fluctuating input voltages, but to show one approach to raster size control which becomes a problem in the presence of input drive signal fluctuations.
These and other problems encountered in the prior art are avoided by the present invention which provides a non-degraded video presentation in a television receiver in the presence of reduced input signal levels caused by utility line level fluctuations due to "brownouts", home appliance start-up, temporary increased utility loads, or any other large drain on a utility power line.